noun pl. keys

1. an instrument, usually of metal, for moving the bolt of a lock and thus locking or unlocking something
2. any of several instruments or mechanical devices resembling or suggesting this in form or use; specif.,
   1. a device to turn a bolt, etc. a skate key, a watch key
   2. a pin, bolt, wedge, cotter, or similar device put into a hole or space to lock or hold parts together
   3. something that completes or holds together the parts of another thing, as the keystone of an arch or a roughened surface forming a secure base for plaster
   4. any of a set of levers, or the disks, buttons, etc. connected to them, pressed down in operating a piano, accordion, clarinet, typewriter, linotype, word processor, etc.
   5. a device for opening or closing an electric circuit
   6. a small metal piece for fastening a wheel, pulley, etc. to a shaft
   7. a key-shaped emblem presented as an honor the key to the city
3. something regarded as like a key in opening or closing a way, revealing or concealing, etc.; specif.,
   1. a place so located as to give access to or control of a region Vicksburg was the key to the lower Mississippi
   2. a thing that explains or solves something else, as a book of answers, the explanations on a map, the code to a system of pronunciation, etc.
   3. a controlling or essential person or thing
4. tone of voice; pitch
5. 1. tone or style of thought or expression in a cheerful key
   2. relative intensity of feeling low-key remarks on a volatile subject
6. the tone of a picture with regard to lightness or darkness or intensity of color
7. ☆

   Etymology: < keyhole, its former shape

   Basketball either of the marked or painted areas on the court near each basket, extending from the end line to the top of the circle that surrounds the foul line
8. Biol. an arrangement or listing of the significant characteristics of a group of organisms, used as a guide for taxonomic identification
9. Bot. key fruit
10. Comput. a field in a record, used to uniquely identify that record
11. Music
    1. Obsolete the keynote of a scale
    2. a system of related notes or tones based on and named after a certain note (keynote, tonic) and forming a given scale; tonality
    3. the main tonality of a composition

Etymology: ME keye < OE cæge, akin to OFris kei, kēia, to secure, guard

adjective

transitive verb keyed, keying key′·ing

1. to fasten or lock with a key or wedge
2. to furnish with a key; specif.,
   1. to put the keystone in (an arch)
   2. to provide with an explanatory key
3. to regulate the tone or pitch of
4. to bring into harmony or accord
5. keyboard